During melting and refining of steel, the gas leaving the furnace carries a substantial amount of fine dust particles. To protect the atmosphere, these particles are removed in cyclones, scrubbers, and other similar devices. The dust particles are made up of two types of materials. Firstly, the dust consists of fine particles charged to the furnace, such as those of iron oxides, calcium oxide, and silica, which are mechanically carried away by the gas. Secondly, the dust consists of oxide particles of nonferrous metals, such as zinc and lead, which evaporate at the high temperatures of the steel bath and later condense and oxidize in the flue. The amounts of these nonferrous metal oxides in the dust increase as the dust is repeatedly recycled or as more scrap containing these metals are charged into the furnace.
The dust can be repeatedly recycled only for a limited number of cycles. With each recycle, the amount of dust to be collected increases due to larger proportion of the very fine dust particles in the feed. The feeding of such fine particles of low bulk density becomes increasingly difficult. Furthermore, as the amount of recycled dust increases, the energy requirement for reducing and melting iron and other metals from the dust increases.
Also, with repeated recycling, the nonferrous metal content of the dust increases to the point where their return to the steel bath adversely affects the steel melting operation. As oxides present in the slag, they attack the refractory lining of the furnace. In reduced form, these metals enter the molten steel at concentrations higher than permissible to meet specifications for proper mechanical and physical properties of the steel. At this point, the dust has to be removed from the system. Traditionally, steel plant dust has been considered a waste material and is generally disposed of in landfills.
However, such disposal methods are becoming more expensive, due to increasing land and transportation costs and due to increasingly stringent environmental protection regulations. Since the dust particles are very fine, they are likely to become airborne unless they are agglomerated to some extent or kept covered. Furthermore, the chemical nature of these dust particles are such that they classify as hazardous waste, based on the EP toxicity test prescribed by the U.S. Environmental Protection Agency. As disclosed in the article entitled "Characterization of Steelmaking Dusts from Electric Arc Furnace", United States Bureau of Mine's Report of Investigation No. 8750 (1983) by S. L. Law et al., the toxicity is related to the presence of lead, cadmium, and sometimes chromium.
In view of the above, there has been an increasing interest in the development of processes that will permit removal and recovery of nonferrous metals, such as zinc, lead, and cadmium, from steel plant dusts. The dust, depleted of these metals, can be recycled back to the steel furnace for recovery of iron or can be stored as a nontoxic waste. Since the tonnage of this material is substantial (about 500,000 tons was produced in 1979 from U.S. electric furnace operations only, according to the aforementioned article), it represents an important source of zinc, lead, and iron metals.